Human papillomavirus (HPV) contains a circular double-stranded DNA genome of about 7,900 bp, which is organized into three main regions, i.e.:                an early coding region containing genes E1, E2, E4, E5, E6 and E7, which are involved in viral replication and in neoplastic transformation,        a late coding region, containing genes L1 and L2, which code for viral capside proteins,        a non-coding regulatory region, which is referred to as LRC (Long Control Region), which is located between the E genes and the L genes.        
HPV constitute a group of viruses, which are associated with benign and malignant lesions of cutaneous or mucosal epithelia. To date, more than 100 different HPV types have been identified.
More than 40 HPV types belonging to the mucosal group have been detected in the anogenital mucosa.
HPV is the major risk factor in the development of squamous intraepithelial lesions (SILs), which are classified as low grade (LSIL) or high grade (HSIL) in severity.
HPV may induce cervical intraepithelial neoplasia (CIN), ranging from benign lesions (CIN1), such as condylomata acuminata, through pre-cancerous lesions (CIN2 to CIN3), up to in situ carcinoma and invasive cancer.
It is now established that HPV is directly involved in cervical carcinogenesis. Detecting HPV is essential to the prognosis of CIN and cervical cancer.
Early and precise detection of HPV is the key factor for recovery from cervical cancer.
It has also been shown that an increased HPV viral load within a cervical smear to specimen is associated with an increased risk of CIN3 and of cervical carcinomas.
A number of oncogenic HPV genotypes that infect the anogenital tract have been classified as potentially high risk HPV genotypes (HR HPV), based on their occurrence or prevalence in cervical carcinomas. The presence, persistence and/or re-occurrence of HR HPV is a bad prognostic indicator. So far, thirteen HPV types are said to be HR HPV, namely HPV 56, 51, 58, 33, 52, 35, 31, 16, 68, 39, 59, 45 and 18. Those HR HPV, which have the highest prevalence, are HPV types 33, 31, 16, 45 and 18.
Other oncogenic HPV are considered to be Low Risk HPV (LR HPV), e.g., HPV2, HPV3, HPV6, HPV11, HPV13, HPV32, HPV40, HPV42, HPV43, HPV44, HPV57.
The clinical classification of HPV types into either the HR or the LR group might evolve, or slightly diverge from one author to another, as the classification of a given HPV into the LR group only stands for as long as this HPV type is not found to be associated with a cervical carcinoma.
For example, it is now contemplated that HPV53 and HPV66 probably are HR HPV (van Ham et al. 2005, J. Clin. Microbiol. Vol. 43, n°6, p. 2662-2667). Hence, the initial group of thirteen HR HPV might further increase to a number of at least 15 HPV types.
Other HPV have been described as oncogenic HPV, but without any definitive settlement on the issue of their HR or LR status, such as is the case for HPV67, HPV82, HPV85. Appropriate detection means are required to analyze their oncogenicity.
New, or yet unidentified, mucosal oncogenic HPV types may further arise.
Furthermore, HPV multi-infection, involving several types of HPV, is a common situation: multi-infection is thought to account for about 20% of the HPV infection cases. An HPV multi-infection case may involve HPV types, which all are oncogenic HPV, or which comprise at least one oncogenic HPV and at least one non-oncogenic HPV. An HPV multi-infection case may involve HPV types, which all are mucosal HPV types, or may involve at least one mucosal type and at least one cutaneous type.
Also, co-infection may also occur, which involves at least one HPV and at least one virus other than HPV, e.g., a co-infection with at least one HPV, and at least one HIV.
Such multi- and/or co-infection situations render accurate HPV detection much more difficult.
HPV primers and probes, which are suitable for the detection of mucosal oncogenic HPV, have been disclosed in prior art.
The first techniques that were developed involved type-specific probes, which were designed to detect oncogenic HPV by direct hybridization of a type-specific probe to a non-amplified HPV genome, e.g., by Southern blotting or dot blotting.
Signal-amplified tests have then been developed, such as the Hybrid Capture test (HC2®) of Digene Corporation, Gaithersburg, Md., USA. The HC2® test has been approved by the FDA, and is at present time the reference test for clinical diagnosis.
The HC2® system is a liquid phase microplate system using DNA/RNA hybridization assay, which does not comprise any target amplification: viral DNA hybridizes in liquid phase to a RNA probe which targets the 13 HR HPV, the hybrids thus formed being detected by anti-DNA/RNA antibodies and visualized by chemoluminescence.
The HC2® test is a sensitive assay. It however is only of qualitative value. Viral loads assessed by the HC2® test does not increase with increasing grade of SIL, and are not sufficiently reliable in case of multiple HPV infections. Hence, the HC2® test is not a quantitative assay.
As the HC2® test does not reflect the viral load initially contained in the analyzed sample, it is recommended to combine it with classic cytology, to distinguish the cases with high grade lesions from those without high grade lesions.
Amplification methods have then been developed, wherein HPV target(s) is(are) amplified by at least one primer pair, the amplicon thus produced being detected either by this (labelled) primer pair or by a probe.
Such prior art primers have first been designed as general consensus primers, which are intended for amplifying several HPV, usually several of the thirteen HR HPV, as well as other HPV (oncogenic LR, and sometimes also non-oncogenic HPV).
Such consensus primers are also referred to as “universal” primers. These consensus primers target conserved regions in the HPV L1 gene (e.g., the MY09/MY11/HMB01 primers, the GP5+/GP6+ primers, the PGMY09/PGMY11 primers, and the SPF1/SPF2 or SPF10 primers), or the E1 ORF region (e.g., the CPIIG/CPI primers described in Tieben et al. 1993, J. Virol. Methods 42:265-279).
To render consensus PCR applicable to clinical diagnosis, HPV probes have been developed to detect and type HPV amplicons generated by consensus primer sets. Detection of the HPV amplicons generated by consensus primers is usually performed by a reverse hybridization line blot assay, or by calorimetric microtiter plate-based enzyme immunoassay.
Illustrative of such consensus PCR methods are the INNO-LiPA HPV test (Innogenetics, Gent, Belgium), and the Amplicor HPV test (Roche Molecular Systems, Branchburg, N.J., USA).
The INNO-LiPA HPV test is a reverse hybridization line probe assay, the prototype research version of which has been described in Kleter et al., 1999 (Journal of Clinical Microbiology, vol. 37, n°8, p. 2508-2517), and Kleter et al. 1998 (American Journal of Pathology, vol. 153, n°6, p. 1731-1739),
Briefly, a PCR primer set is used to generate a short PCR fragment (SPF PCR) of 65-bp from the L1 open reading frame. The prototype research INNO-LiPA primer set consists of 10 biotinylated primers (referred to as the SPF10 primer set), namely the six primers of the SPF1/2 system (described in Kleter et al. 1998), and four additional primers (MY09/11 and GP5+/6+).
The SPF10 amplimers are denatured, and incubated under hybridization conditions with poly(dT)-tailed type-specific oligonucleotide probes, which are immobilized as parallel lines on nitrocellulose membrane strips. The probe strips are then washed out for detection of the retained hybrids.
The INNO-LiPA HPV test allows the detection of at least 25 HPV genotypes (the 13 HR HPV, i.e., HPV 56, 51, 58, 33, 52, 35, 31, 16, 68, 39, 59, 45, 18, as well as other HPV, e.g., HPV 6, 11, 34, 40, 42, 43, 44, 53, 66, 70, and 74). It is a genotyping line probe assay, and is of qualitative value. The INNO-LiPA HPV test is not a quantitative assay.
The Amplicor HPV test uses amplification of target HPV DNA by PCR followed by nucleic acid hybridization for the detection of the thirteen HR HPV. The Amplicor HPV test amplifies a sequence of about 165 bp within the L1 region. The primer sets consist of 12 primers, which have been designed as general consensus primers, to amplify the initial group of 13 HR HPV. After amplification and denaturation, the amplified HPV sequences are distributed in a microwell plate, and incubated with L1 capture probes, the hybrids being detected and visualized by colorimetric enzyme immunoassay (avidin-horseradish peroxidase conjugate).
The Amplicor HPV test has been reported as being of higher analytical specificity, compared to the HC2® test (less false negative results, see Poljak et al. 2005, Acta Dermatoven APA, vol. 14, n°4, p. 147-152).
The Amplicor HPV test is sensitive, but its HPV spectrum is restricted to those 13 HPV, which have been initially considered as being the HR HPV. For example, the Amplicor HPV test does not detect HPV66 and HPV53, which are now thought to be HR HPV. In other words, the Amplicor test is not designed to be adaptive to any change or evolution in HPV classification or knowledge.
Furthermore, the Amplicor HPV test is not a quantitative assay.
These line blot or microwell-based prior art techniques use consensus HPV primers, i.e., primers which result from sequence alignment of a pre-determined set of selected oncogenic HPV, and from the determination of those consensus sequences, which have a sufficient similarity or identity score with all of the selected HPV, to hybridize to all of them. Consensus primers are thus designed to amplify a predetermined sub-set of oncogenic HPV, and may not succeed in amplifying other oncogenic HPV (such as HR new corners, or non-HR oncogenic HPV).
Such a consensus approach is restricted by the possibility of determining primer sequences, which would still sufficiently hybridize to the ever increasing and ever evolving desired targets.
If one or several new oncogenic HPV strain(s) appear, such prior art consensus primers might give false negative results.
Also, none of the prior art line blot or microwell-based techniques is of a quantitative nature, whereas recent findings show that the HPV copy number accounts for the phase and/or severity of the disease, and/or have a diagnostic and/or prognostic value in the field of oncogeny.
Absence of quantitative performance limits the spectrum of clinical applicability, as such tests cannot give information on the actual cancer risk level, or on the actual cancer grade.
Moreover, according to these prior art consensus line blot or microwell-based techniques, the detection step is an additional and tedious step, which has to be performed as a separated step after amplification has occurred.
Real-time PCR amplification techniques have recently been developed for the detection of HPV16 or HPV18 (Hesselink et al. 2005, Journal of Clinical Microbiology, vol. 43, n°9, p. 4868-4871; Gravitt et al. 2003, Cancer Epidemiology, Biomarkers & Prevention, vol. 12, p. 477-484).
These real-time PCR either are based on FRET (LightCycler), or use TaqMan probes (Applied Biosystems).
Compared to prior art line blot or microwell-based techniques, such real-time PCR have the advantage of combining amplification and detection in one single step, and of opening the way to quantification.
For example, van Duin et al. 2002 (Int. J. Cancer, vol. 98, n°4, p. 590-595) describes a quantitative real-time PCR assay for the detection of HPV16.
Prior art real-time PCR protocols however are type-specific PCR protocols, which are limited to the detection of only one HPV per amplification run, and more particularly to the sole detection of HPV16 or HPV18. They thus represent valuable research tool, but have a very limited clinical applicability.
Attempts have been made to develop multiplex real-time PCR amplification of HPV. These attempts are however limited to duplex or triplex real-time PCR for the detection of HPV16, HPV18, HPV45. For example, Szuhai et al. 2001 (American Journal of Pathology, 159(5): 1651-1660) disclose seven type-specific to molecular beacons, which are said to be type-specific molecular beacons, namely five HR HPV molecular beacons (HPV16, 18, 31, 33, 45) and two LR HPV molecular beacons (HPV6, 11); see table 1 of Szuhai et al. These molecular beacons are described as being useful for the detection of amplicons generated by the CPI/CPIIG “universal” primers. Multiplex attempts are disclosed in Szuhai et al., but are limited to duplex or triplex assays (HPV16, HPV18, HPV45). The authors explicitly indicate that “although the multiplexing capacity of molecular beacon PCR is higher than three, it is unlikely that it will approach the number of different HPV genotypes” (see page 1656, right-hand column, second paragraph). For this reason, the authors came to the conclusion that type-specific molecular beacons cannot by their own solve the problem of HPV clinical diagnosis, and that they shall be used in combination with a general pre-screening HPV detection method, to arrive at a two-step HPV detection and genotyping strategy (see e.g., FIG. 6 of Szuhai et al.), wherein type-specific HPV molecular beacon PCR is disclosed to be used in combination with a SybrGreen general primer PCR pre-screening.
Hence, to the best of the inventors' knowledge, prior art does neither describe nor suggest any real-time amplification technique that could be worked in multiplex, whilst retaining the required HPV detection specificity, which would allow to cover at least the 13 HR HPV in a single step (amplification+detection) run. Furthermore, to the best of the inventors' knowledge, prior art does not describe any quantitative real-time HPV amplification technique, which would allow to cover at least the five most common HR HPV (namely, HPV16, 18, 45, 31 and 33), preferably at least the 13 HR HPV, more preferably at least the 13 HR HPV as well as five other oncogenic HPV, in a single step (amplification+detection) run, and which would be quantitative, even when implemented in multiplex.